1. Field of the Invention
This invention pertains to devices incorporating dispersive volume transmission gratings, and more particularly to Raman and/or fluorescence spectrometers that have continuous tuning of the excitation wavelength.
2. Discussion of Related Art
Holographic volume transmission gratings are used in a variety of devices and are proven to have high efficiency over a wide spectral range. (For example, see, Tedesco et al, U.S. Pat. No. 5,011,284, the entire contents of which are incorporated herein by reference.) Unlike the situation with widely used reflection surface-relief gratings, the directions of the incident and efficiently diffracted beams are symmetrical with respect to the periodicity of the grating with volume transmission gratings. Wavelength tuning in spectrometers with reflection gratings can be performed by rotating the grating in a Littrow-like configuration. In George et al. U.S. Pat. No. 4,752,130, the entire contents of which are incorporated herein by reference, spectrometers, monochrometers and other devices are described that use volume transmission gratings. However, the devices described in George et al require individually moving at least one grating and at least one mirror with complex devices to match the diffracted and/or incident beam directions, determined by Bragg""s conditions. The resultant optical layouts described in George et al are too cumbersome to have significant practical applications. Instead, commercially available spectrometers (e.g., HOLOPROBE made by KAISER OPTICAL SYSTEMS, INC.) utilize xe2x80x9csnap-inxe2x80x9d gratings. Other conventional devices have stacks of gratings for changing the spectral range of coverage. Such devices are not practical for many applications, including scientific research applications, in which flexible changes of both the excitation wavelength and the spectral range are essential.
For example, flexible changes of both the excitation wavelength and the spectral range are important for spectrometers used for analyzing secondary radiation emitted by a sample under primary excitation by a laser or other source of radiation when it is necessary to distinguish between Raman and fluorescence signals. The Raman signal has essentially the same wavenumber shift with respect to the excitation frequencies, while the luminescence one preserves the positions of the bands on the wavelength scale. Thus, measurements with two or more different excitations permit one to sort out the Raman and fluorescence signals. Moreover, in many cases, the Raman intensities depend critically on the excitation wavelength (resonance Raman), and provide information about electronic and other properties of the sample. Similar information can be obtained from the fluorescence excitation spectra, by measuring the intensity of a particular band as a function of the excitation wavelength. Also, wavelength tuning in a wide spectral range is necessary when measuring the optical properties of substances to investigate their electronic properties.
Therefore, there is a need for improved wavelength tunable devices, such as improved wavelength tunable spectrometers and spectrographs. The conventional devices used for measurements of emission, absorption or reflection spectra in a wide spectral range are surface relief single-grating spectrographs with a CCD array detector. Although adequate for some applications, this kind of conventional device is large and bulky when high spectral resolution is required, e.g., 0.1 nm or higher, because of the necessity to increase the focal length of the spectrograph, and also normally has a decreased throughput resulting from such physical limitations.
Some conventional spectrographs that use reflection gratings have a very important feature that allows the user to change the spectral coverage (and spectral resolution, concomitantly) rapidly without any realignment, thus preserving the calibration. This is realized by having two or more gratings on the same rotation turret, driven by a computer. However, current spectrographs with volume gratings use a different principle: they have either snap-in gratings, or they have a stack of gratings dispersing the spectra on different strips of the CCD shifted in a direction perpendicular to the spectral direction. In the first case, recalibration is needed after each change of the grating, while in the second case, throughput loss results. Neither method is flexible enough for many applications, including scientific measurements, because a change of the central wavelength would require a different grating.
Prism-based selecting elements are widely used in practice for laser intra-cavity wavelength selectors and laser monochrometers. Prism-based selecting elements have high transmission and can be wavelength-tuned, but because of very limited dispersion, prism filters are inadequate for many applications. For example, in the case of Raman spectroscopy, low-frequency laser plasma lines (below 100-200 cmxe2x88x921) leak through the system and appear in the Raman spectra as spikes, which can mask the useful Raman signal. Surface-relief grating monochrometers serve adequately in some cases, but they have several problems. Aberrations originating from their off-axis spherical collimating mirror optics cause significant problems. Thus, using surface-relief gratings in the part of the system delivering the laser beam (laser filters and beam splitters) would deteriorate the quality of the laser spot on the sample and, consequently, the spatial resolution of the device. The throughput of grating monochrometers is polarization and wavelength-dependent and normally does not exceed 50%. Grating monochrometers are also bulky, and in the case of the double-subtractive monochrometer, which is used as the laser-rejecting stage for conventional Raman spectrometers, require accurate and time-consuming alignment. The same is true for the use of a grating monochrometer as a laser beam-splitter. Consequently, although the use of surface relief grating optics is adequate for some laser and nonlaser spectroscopic applications, they are not adequate for devices that require a rapid change of excitation and/or spectral range, they are bulky and complicated, and they are not sufficiently efficient for many applications.
In the case of Raman/fluorescence/excitation spectrometers, it is extremely important to filter out the excitation radiation so that none of the unwanted radiation (e.g., plasma lines of the ion laser tube) is present as spurious bands in the measured spectra. Another important feature of these devices is to provide a way to inject the excitation radiation into the optical system and then to reject the excitation radiation before the spectrograph stage (i.e., analysis of the spectrum). Different types of filters are currently in use for cleaning the laser radiation. Simple color glass filters are adequate for non-demanding applications such as for observing fluorescence spectra or high-frequency Raman spectra. In this case, a neutral beam splitter can be used for injecting the laser radiation and a color glass rejection filter for removing the laser radiation from the signal. Since color glass filters have a very broad edge between the transmission and absorption spectral range, the use of this configuration is very limited. Interference filters and beam splitters offer more of an abrupt edge for both edge and notch type dichroic filters.
There are two types of interference filters: multilayer thin dielectric films with different refractive indexes deposited between two highly reflective layers (i.e., the Fabry-Perot principle) and, alternatively, low/high refractive index periodic structures produced by laser interference in photosensitive materials (i.e., the xe2x80x9cholographicxe2x80x9d technique). In the latter case, because the refractive index can be changed smoothly with respect to the coordinates of the medium, much sharper changes in the transmission/reflection coefficient can be achieved with much better rejection close to the excitation wavelength. Dielectric interference filters are widely used in Raman spectroscopy as laserline band-pass filters and also as signal band-pass filters for Raman imaging. Dielectric interference filters also can be manufactured as step-like short-pass and long-pass types to pass and/or reject the laser excitation, but because of relatively long transmission tails, sufficient rejection for Raman spectroscopy cannot be achieved and the low-frequency spectral range remains unavailable.
Holographic filters (volume gratings) offer much better performance compared to dielectric filters. There are two major types of volume gratings: conformal reflection and untilted fringe transmission holograms, which differ by orientation of the fringes relative to their respective surfaces. (See, Tedesco et al, Principles and Spectroscopic Applications of Volume Holographic Optics, Analytical Chemistry, 65, 441A-449A (1993), the entire contents of which are incorporated herein by reference.) The first type of volume grating is nondispersive and is widely used in practice as a notch filter with up to 6 O.D. Non-dispersive volume gratings have superior performance compared to dielectric interference type filters. (See, Yang et al, Holographic Notch Filter for Low-wavenumber Stokes and Anti-Stokes Raman Spectroscopy, Applied Spectroscopy, 45, 1533-1536 (1991); and Schoen et al, Performance of Holographic Supemotch Filter, Applied Spectroscopy, 47, 305-308 (1993). The entire contents of both are incorporated herein by reference.) A nondispersive volume grating has also been used as a laser beam splitter in Da Silva et al, U.S. Pat. No. 5,661,557, the entire contents of which are incorporated herein by reference. The spectral position of the notch depends on the incident angle and can be adjusted by rotation of the filter in such a way that the laser wavelength matches the spectral range of minimum transmission, but the filter is transparent to as low of a portion of the Raman signal as possible. The use of the notch filter as a beam splitter is normally limited to one excitation wavelength because using it for the next excitation wavelength (e.g., of the argon ion laser) would require rotation of the notch filter by a large angle, so that it becomes impractical. Such conventional devices thus require changing the notch filter for every change of the excitation wavelength.
The second type of holographic grating is a dispersive element (volume diffraction grating). Accordingly, in view of the above-noted problems, the present invention provides new and improved devices using dispersive volume diffraction gratings.
It is an object of the present invention to provide devices which incorporate dispersive volume transmission gratings for spectroscopic applications which are compact.
It is another object of this invention to provide devices that incorporate dispersive volume transmission gratings in devices that are efficient and conveniently wavelength tunable.
It is another object of this invention to provide a Raman spectrometer that is efficient and compact, and is conveniently tunable over excitation wavelengths.
A preferred embodiment of this invention relates to a spectrometer which has a source of illumination radiation, a dispersive beam splitter disposed in an optical path of light from the source of illumination radiation, and a spectrograph disposed in the path of radiation from a sample illuminated by the illumination radiation from the source. The dispersive beam splitter is tunable to disperse a selected wavelength component of the illumination radiation from the source of illumination radiation. In particular, the dispersive beam splitter according to a preferred embodiment of the invention has a wavelength tunable dispersion assembly that includes a volume dispersion diffraction grating and a mirror which are rotatable together substantially as a unit about a common axis of rotation. In a preferred embodiment, the spectrometer also has an optical bandpass filter arranged between the source of illumination and the dispersive beam splitter. In a second preferred embodiment of a spectrometer according to the invention, a monochrometer replaces the optical bandpass filter of the first preferred embodiment.
The invention is also directed to a wavelength tunable dispersive beam splitter which has a light input port, a light output port and a wavelength tunable dispersion assembly disposed between the light input and the light output port. The wavelength tunable dispersion assembly has a mirror and a volume dispersion diffraction grating which are substantially fixed in orientation and displacement with respect to each other and are rotatable together substantially as a unit with respect to a common axis of rotation.
The invention is also directed to an optical bandpass filter that has a light input port, a light output port and a wavelength tunable dispersion assembly disposed between the light input port and the light output port. The wavelength tunable dispersion assembly has a mirror and a volume dispersive diffraction grating which are substantially fixed in orientation and displacement with respect to each other. The volume dispersive diffraction grating and the mirror are rotatable together substantially as a unit with respect to a common axis of rotation.
The invention is also directed to a spectrograph that has an input aperture, a detector arranged in a substantially fixed orientation and displacement with respect to the input aperture and a wavelength tunable dispersion assembly. The wavelength tunable dispersion assembly of the spectrograph has a mirror and a volume dispersion diffraction grating that are substantially fixed in orientation and displacement with respect to each other. The volume dispersive diffraction grating and the mirror are rotatable together substantially as a unit with respect to a common axis of rotation.
The invention is also directed to a monochrometer that has a light input aperture and a wavelength tunable dispersion assembly proximate the light input aperture. The wavelength tunable dispersion assembly has a mirror and a volume dispersive diffraction grating that are substantially fixed in orientation and displacement with respect to each other. The volume dispersive diffraction grating and the mirror are rotatable together substantially as a unit with respect to a common axis of rotation. The invention is also directed to a combination of monochrometers, as described above, arranged in series to increase dispersion or further suppress the elastically scattered or reflected radiation from the primary illumination radiation source.